Services on a communication network have become increasingly diversified, and new services taking advantage of the network have been expanding. A representative example thereof is the integration of broadcasting and communication services such as so-called triple-play service for integrating broadcasting, Internet, and telephone (audio communication) services. To achieve the triple-play service, FTTH (Fiber To The Home) construction with PON is becoming mainstream in an access network. In this PON system, plural subscribers share an optical fiber from a subscriber accommodation office to an optical splitter and facilities in the office, which leads to cost sharing for reductions in initial introduction cost and maintenance cost. In the PON-based FTTH system which is a shared-media network described above, a bandwidth available to a subscriber is roughly equal to a value obtained by dividing the maximum throughput of the system by the number of sharing subscribers. However, due to a low probability of simultaneous access by all subscribers, the subscriber can practically use a greater bandwidth by statistical multiplexing effect. With such a PON-based wideband FTTH system, it is possible to achieve comfortable triple-play service. Current systems include ITU-T G-PON and IEEE GE-PON. Details of G-PON are defined, for example, in ITU-T G.984.1 “Gigabit-capable Passive Optical Networks (GPON): General characteristics”, ITU-T G.984.2 “Gigabit-capable Passive Optical Networks (GPON): Physical Media Dependent (PMD) layer specification”, and ITU-T G.984.3 “Gigabit-capable Passive Optical Networks (GPON): Transmission convergence layer specification”, and details of GE-PON are defined in IEEE 802.3ah “CSMA/CD Access Method and Physical Layer Specifications Amendment: Media Access Control Parameters, Physical Layers, and Management Parameters for Subscriber Access Networks.” For example, in the G-PON system, an optical line terminating apparatus (OLT) accommodates up to 64 optical network terminating units (ONUs) through a 2.4-Gbps high-speed optical line. Collision avoidance control is one of the schemes of sharing facilities in the office as described above. Optical signals (upstream signals) are outputted from ONUs to the OLT, with the optical signal powers being mutually superposed by an optical splitter. In order for the OLT to receive plural separate signals, there is performed transmission timing control such that the signals from the ONUs arrive at the OLT at different times without being mutually superposed, that is, collision avoidance control. Currently, standardizing organizations (ITU-T and IEEE) have started to study next-generation PONs subsequent to these current PON systems. For a wider band of the PON system, studies are being conducted on a higher speed of TDMA applied to the current PON, an increase in bit rate, and the like.
The wavelength multiplexing transmission technique is applied to the triple-play service in the PON system. A wavelength range of 1550 to 1560 nm is allocated to a video transmission system. In the PON system, 1490 nm band data signal light is allocated to downstream optical signals from the OLT to the ONU, and 1300 nm band data signal light is allocated to upstream optical signals from the ONU to the OLT. It is desirable that a next-generation PON targeted at a communication speed of 10 Gbps share a fiber with the existing GE-PON and G-PON systems for system construction.
However, in an optical transmission system having a bit rate of 10 Gbps, a phenomenon called the wavelength dispersion of an optical fiber greatly limits transmission speed and transmission distance. The wavelength dispersion is a phenomenon in which beams having different wavelengths propagate at different speeds in the optical fiber. Since the optical spectrum of an optical signal modulated at high speed includes different wavelength components, these components arrive at a receiver at different times due to the effect of the wavelength dispersion when propagating through the optical fiber. This causes distortion in optical signal waveform after fiber transmission. There is a technique called dispersion compensation to suppress the waveform degradation due to the dispersion. The dispersion compensation is a technique in which an optical element having a wavelength dispersion characteristic inverse to that of an optical fiber used as a transmission line is disposed in an optical transmitter, a receiver, or a repeater so as to cancel the wavelength dispersion characteristic of the optical fiber to prevent the waveform degradation. As this optical element, that is, a dispersion compensator, devices having an inverse dispersion characteristic such as a dispersion compensation fiber and an optical fiber grating have been studied and put to practical use. However, the dispersion compensator is expensive and not practical for the PON system. As a method not using the dispersion compensator, there is a method of using a low-chirp external modulator. The chirp is a minute and dynamic wavelength variation which occurs when an optical carrier emitted from a communication laser is modulated in an optical communication system. The chirp causes group delay according to a wavelength dispersion value of the optical transmission line, thus distorting an optical signal pulse waveform and degrading transmission quality. For a wavelength of 1490 nm or greater used in the PON system, in the case of directly modulating a laser, it is difficult to achieve a transmission distance of 20 km under the influence of the chirp and the dispersion. In this case, it is considered that a method of using EA (Electro-Absorption) modulator with the electroabsorption effect of a semiconductor is favorable. This is because, EA is made of semiconductor material, which facilitates the integration of the external modulator and the laser, thereby making it possible to suppress a cost increase compared to a modulator using optical crystal having electrooptic effect such as LiNbO3. Compared to the method of directly modulating a laser, the use of the EA modulator causes a cost increase corresponding to the modulator. However, in the PON system, plural subscribers share facilities in the office for cost sharing; therefore, in the case where the modulator is used in the optical line terminating apparatus, this cost increase is not a fatal problem.